DAMASCUS, SYRIA - APRIL 07: An affected Syrian man lies on a stretcher as he waits to get medical treatment after Assad regime forces allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta in Damascus, Syria on April 07, 2018. (Photo by Halil el-Abdullah/Anadolu Agency/Getty Images)

IDLIB, SYRIA - APRIL 08: A man holds a photo of a victim baby as he and other demonstrators gather to protest against Assad regime forces' allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta, in Saraqib town of Idlib, Syria on April 08, 2018. (Photo by Mahmut Bekkur/Anadolu Agency/Getty Images)

Activists and aid groups say dozens died in the assault on Douma, the last rebel stronghold in eastern Ghouta, which has been subjected to intensive bombing by Syria's Russian-backed regime of President Bashar al-Assad.

The samples suggested the presence of both chlorine gas and an unnamed nerve agent, two officials said. Typically, such samples are obtained through hospitals and collected by U.S. or foreign intelligence assets on the ground. The officials said they were "confident" in the intelligence, though not 100 percent certain.

Assad is known to have stocks of the nerve agent sarin, and has previously used a mixture of chlorine and sarin in attacks, according to U.S. officials.

Lavrov urged more diplomatic contacts over the issue and said Russia was open to further telephone contact between Trump and President Vladimir Putin.

"Diplomatic methods need to be used, which do not include ultimatums and threats," he said.